1. Field of the Invention
This invention relates to a current limiting fuse having external means for actuating or initiating the operation of the fuse.
2. Description of the Prior Art
Two types of current limiting fuses for use with transformers are generally employed. One fuse is a full-range fuse connected in the primary circuit. The other fuse is a limited range back-up fuse in series with an under-oil expulsion link connected to the primary. Both of these fuses have limitations.
The full-range device has a melting time-current characteristic far too steep to coordinate with the usual transformer damage curve. The fuse is physically large and, at higher voltages, it has a limited current rating. Moreover, a full range fuse of contemporary design cannot successfully interrupt low currents which may occur when, for example, an unloaded transformer melts the fuse on inrush and then falls back to the exciting current level.
The link and back-up fuse combination provides a more desirable solution, because the link more nearly approximates the sloping long-time damage characteristic of the transformer and is capable of extinguishing relatively small over-currents. The back-up current limiting fuse clears higher current faults above the capability the link. However, one limitation involves a difficulty of successfully coordinating the fuse with the link at the desired current level. Another limitation is that an oil-immersed link precludes the drawout well utilization now coming into prominence in transformer usage.
Prior art patents which are generally related to the subject matter of this invention include the following U.S. Pat. Nos.: 2,599,646; 3,256,409; 3,743,994; 3,825,870; 3,840,836; 3,911,385; 3,925,745; 3,949,342; and 4,028,655.